1. Field of the Invention
Implantable defibrillator cardioverter systems are the broad area that this invention concerns, with the specific focus being on methods for detecting the presence of ventricular fibrillation promptly and accurately, and for distinguishing fibrillation from tachycardias.
2. Description of the Prior Art
A major challenge in the defibrillation art is the prompt and accurate detection of the ventricular fibrillation condition. Fibrillation is the rapid, but unsynchronized, contraction of heart-muscle elements, causing the blood-pumping action of the heart to diminish seriously or even to cease. It clearly involves a departure from the pulse rate of normal sinus rhythm (NSR), 60 to 120 beats per minute, but is not unique in this respect because other and quite different conditions can also cause pulse-rate departures from NSR. The challenge thus is to define a reliable criterion for identifying the condition of ventricular fibrillation. Pulse rate alone is not a dependable criterion, even though it is often used for the purpose. Lacking a reliable criterion, there are the twin hazards of failing to detect the onset of fibrillation, and of applying a fibrillation shock when such treatment is not appropriate. The first of these lapses is fatal, and the second is painful and disconcerting, as well as potentially dangerous. Furthermore, the latter event represents a waste of precious and limited energy in the case of an implanted defibrillation system.
Conditions distinct from ventricular fibrillation that involve a rapid pulse, above 120 per minute, are identified by the generic term tachycardia. Further complicating the picture, however, is the fact that there are different kinds of tachycardia. Monomorphic ventricular tachycardia (MVT) involves degraded coordination in the contraction of the ventricle, but not the chaotic behavior found in fibrillation. It is a hazardous condition, and can be treated effectively in many cases by cardioversion, a shock with energy in the neighborhood of one joule, significantly less than is used in a typical defibrillation procedure.
Second, the condition of supraventricular tachycardia (SVT) involves better coordination than MVT and is usually not life-threatening. Since its origin is above the ventricular region, it does not respond to the most common cardioverter-defibrillator kinds of treatment that focus their energy delivery on the ventricular region. A type of SVT, known as sinus tachycardia, is caused by emotional or physical stress, and pumping action remains normally efficient so no intervention is necessary or desirable.
Thus, the diagnostic challenge is to distinguish among three conditions or sets of conditions: (1) Intervention is either not needed or not effective in the set comprising the conditions of normal sinus rhythm, sinus tachycardia, and supraventricular tachycardia. (2) A comparatively low-energy shock is appropriate in the case of monomorphic ventricular tachycardia. (3) A high-energy defibrillation shock is indicated in the case of ventricular fibrillation.
Electrical signals generated by the heart muscle are routinely sensed in pacemaker applications. Sometimes these signals are picked up by the pair of electrodes also used for delivering the pacing impulse. Normally these electrodes are near the distal end of a catheter that is introduced intravenously, and positioned at the right-ventricular apex. Typically, a tip electrode is right at the end of the catheter, and a ring electrode is positioned about one centimeter away from the end, as illustrated in FIG. 1.
Also shown schematically in FIG. 1 are two additional coils; electrodes associated with the catheter that are for delivering a defibrillation shock, one electrode located within the right ventricle, and one at the top of the right atrium. Another electrode option for defibrillation are the epicardial patches illustrated in FIG. 2. In either case, the defibrillation electrodes can also be used for picking up electrical heart-waveform signals, in lieu of or in addition to the two pacing electrodes. In still another arrangement, one of the defibrillation electrodes may serve as a common electrode for several purposes in addition to defibrillation--pacing, simple pulse detection, and waveform observation.
The shape, or morphology, of the electrical waveform delivered by the heart changes with the onset of fibrillation. But these changes, unfortunately, are neither consistent enough nor pronounced enough to serve as unsupported fibrillation criteria. One change is a tendency toward an erratic pulse (variable intervals between heartbeats), known commonly as a departure from rate stability. Another change is a tendency for heart voltage to be zero for a smaller fraction of the time, or a change in what is commonly described as the probability distribution function. That is, the signal dwells near the baseline for a smaller fraction of the cardiac period.
Because pulse observation is straightforward, a number of other features of simple pulse rate have been brought into use in an effort to improve the accuracy of fibrillation detection. Among these are rate acceleration, and time at a particular rate. But these indicators are equivocal, and consequently not even this elaborate method avoids "overlap" with the aggregate behavior of these pulse features in a heart functioning normally, or functioning in a manner such that a shock is not helpful. The monitoring of completely different variables, such as blood pH, pressure, and oxygen saturation are subjects of current research, but have not yet reached clinical application. Hence, the present invention seeks to exploit information already partly delivered by existing systems, but thus far unused.